A water-based plaster obtained by spreading a water-based adhesive base containing a water-soluble polymer, a humectant, and water on a support such as a non-woven fabric has been widely distributed in the market as a pharmaceutical preparation for anti-inflammatory and analgesia or a refrigerant.
A method for adjusting the gel strength of a water-based adhesive base by crosslinking carboxyl groups in a polycarboxylic acid and a salt thereof which are water-soluble polymers with aluminum ions generated by dissolving an aluminum compound which is one of base ingredients to be contained in the water-based adhesive base has been often used. The degree of crosslinking is a factor of influencing the adhesiveness of a water-based plaster as a final preparation, and in addition, a factor of significantly influencing the workability in a production process.
The process of producing a water-based plaster is broadly divided into steps of mixing respective base ingredients (kneading step), thinly spreading (coating) the mixed base ingredients on a non-woven fabric or the like (spreading step), cutting a preparation obtained in the spreading step into a desired size (cutting step), and aging the cut preparation, that is, allowing the preparation to stand and storing the preparation until the gel strength is in a steady state by the development of crosslinking reaction between a water-soluble polymer and a crosslinking agent (aging step).
However, when the crosslinking rate of the water-based adhesive base in the water-based plaster is not sufficiently adjusted, the viscosity of a mixture may rapidly increase in the kneading step. Further, in the spreading step, the mixed base ingredients cannot be uniformly spread on the support, to cause spread unevenness (a state where a paste is non-uniformly spread).
In addition, when a support having a high air permeability such as a non-woven fabric is used, a paste permeates the non-woven fabric and exudes from the back side in the aging step. Thus, an undesirable influence in terms of quality may occur.
Therefore, it is very important in the production of the water-based plaster that the degree of crosslinking of the water-based adhesive base is closely adjusted. Various attempts have been conventionally made.
Patent Document 1 discloses a poutic mainly containing an aluminum salt of water-soluble polymer of an aliphatic carboxylic acid such as polyacrylic acid or a salt thereof. Patent Document 1 describes that the reaction rate of the polymer depends on the dissolution rate of a water-insoluble aluminum compound, that is, the surface area and the solubility product of the water-insoluble aluminum compound and the pH of a solution, and that a crosslinking reaction can be controlled by addition of an organic acid having an OH group in the molecule or a salt thereof.
Similarly, Patent Document 2 focuses on a combination of polyacrylic acid and a salt thereof which are water-soluble polymers forming a skeleton of hydrous gel. Patent Document 2 has proposed a hydrous gel plaster base which has a high gel strength from immediately after production, that is, a good processability, by using a monovalent salt of macromolecular polyacrylic acid and a low molecular polyacrylic acid in combination, and a highly water-soluble aluminum salt as a crosslinking agent.
Patent Document 3 discloses a method for producing a hydrous paste, in which in a step of mixing base ingredients, a polyvalent metal salt, a macromolecular compound, water, and the like are mixed in advance under a condition of low pH, and the pH of the paste is then increased during mixing other base ingredients.
However, in the water-based plaster disclosed in Patent Document 1, the gel strength significantly depends on the physicochemical properties of the water-insoluble aluminum compound to be contained. Therefore, a very severe quality control is needed. In addition, the fine mixing adjustment of the base ingredients is required. This is because the crosslinking is not promoted by an excessively large amount of organic acid (oxyacid) having an OH group.
In Patent Document 2, when the highly water-soluble aluminum salt is used as the crosslinking agent, aluminum ions are dissolved from immediately after production. For this reason, as the gel strength is higher and the spreading time is longer, gelation is promoted to remarkably increase the viscosity of the resulting water-based adhesive base. Therefore, Patent Document 2 has a problem in which a time of spreading a paste on a support is limited to a short period of time.
In Patent Document 3, when the viscosity of the hydrous paste is excessively increased in a spreading step, the paste may be non-uniformly spread on the support.